1. Field of the Invention
The present invention relates to a gain-control-type transistor power amplifier. More particularly, the present invention relates to a gain-control-type transistor power amplifier suitable for adjusting an output signal level of a power amplifier for transmission disposed in a transmission and receiving unit on a subphone side of a digital cordless telephone set.
2. Description of the Related Art
Hitherto, digital cordless telephone sets transmit and receive signals on a time-division basis using the same frequency between the base phone and subphones. In particular, on the subphone side, when a signal from the base phone is received, a received signal intensity indicating the received signal level is detected, and when a signal is transmitted, the gain of the transmission power amplifier is controlled according to the detected received signal intensity so that the transmission signal level is controlled, and this controlled signal is transmitted to the base phone side. In this case, when the received signal intensity is increased, control is performed so that the gain of the transmission power amplifier is decreased, and the transmission signal level is decreased. On the other hand, when the received signal intensity is decreased, the gain of the transmission power amplifier is increased, and the transmission signal level is increased so that the consumption of the built-in battery which drives the subphone is reduced as low as possible.
In this case, all the components of the the subphone of the above-described known digital cordless telephone set are formed of small circuit elements, such as transistors. The subphone comprises a transmission section, formed of a transistor power amplifier, a frequency mixer, a filter, and the like, which constitute a transmission power amplifier, a receiving section, formed of a transistor receiving amplifier, a frequency mixer, a filter, and the like, which constitute a receiving amplifier, and a transmission and receiving common section formed of a phase control loop (PLL) including a transmission and receiving select switch, a filter and a voltage controlled oscillator (VCO), and others. These components of the transmission section, the receiving section, and the transmission and receiving common section are formed into one unit and disposed within a transmission and receiving unit (TR unit).
FIG. 4 is a circuit diagram illustrating an example of a gain-control-type transistor power amplifier for use in the transmission section of the known digital cordless telephone set.
As shown in FIG. 4, the gain-control-type transistor power amplifier comprises a pre-amplifying transistor 41 which is emitter-grounded, a power amplifying transistor 42 which is similarly emitter-grounded, an input filter circuit 43 formed of a series resistor 43a, a series inductor 43b and a branch capacitor 43c, first and second coupling capacitors 44 and 45, a first collector load 46 formed of a series-connected inductor 46a, a resistor 46b and a branch capacitor 46c, an interstage filter circuit 47 formed of a series inductor 47a and a branch capacitor 47b, a third coupling capacitor 48, a second collector load 49 formed of a series inductor 49a and a branch capacitor 49b, an output filter circuit 50 formed of a series inductor 50a and a branch capacitor 50b, a fourth coupling capacitor 51, a first base bias circuit 52 formed of a series inductor 52a and a branch capacitor 52b, a second base bias circuit 53 formed of a series resistor 53a and a branch capacitor 53b, a signal input terminal 54, a signal output terminal 55, a power terminal 56 on the high voltage side, a power terminal 57 on the low voltage side, and a gain-control-voltage supply terminal 58.
The pre-amplifying transistor 41 is connected at its base to the signal input terminal 54 via the second coupling capacitor 45 and the first coupling capacitor 44 and further connected to the gain-control-voltage supply terminal 58 via the first base bias circuit 52, and is connected at its collector to the power terminal 56 on the high voltage side via the first collector load 46 and further connected to the base of the power amplifying transistor 42 via the interstage filter circuit 47 and the third coupling capacitor 48. The power amplifying transistor 42 is connected at its base to the power terminal 57 on the low voltage side via the second base bias circuit 53, and is connected at its collector to the power terminal 56 on the high voltage side via the second collector load 49 and further connected to the signal output terminal 55 via the output filter circuit 50 and the fourth coupling capacitor 51.
In this case, a radio frequency signal (hereinafter referred to as an "RF signal") to be amplified is supplied to the signal input terminal 54, and the amplified RF signal is output from the signal output terminal 55. A relatively high power voltage is supplied to the power terminal 56 on the high voltage side, and a relatively low power voltage is supplied to the power terminal 57 on the low voltage side. A gain control voltage which varies in response to the received signal intensity is supplied to the gain-control-voltage supply terminal 58, and the varying range of the gain control voltage is, for example, from 0 to 1.5 V.
The gain-control-type transistor power amplifier constructed as described above operates as described below.
An RF signal to be amplified, which is applied to the signal input terminal 54, is supplied to the base of the pre-amplifying transistor 41 via the first coupling capacitor 44, the input filter circuit 43, and the second coupling capacitor 45, is pre-amplified by the pre-amplifying transistor 41 and output from the collector thereof. At this time, the amplification gain of the pre-amplifying transistor 41 becomes a variable gain, which is dependent on the magnitude of the gain control voltage applied to the gain-control-voltage supply terminal 58. Then, the RF signal pre-amplified by the pre-amplifying transistor 41 is supplied to the base of the power amplifying transistor 42 via the interstage filter circuit 47 and the third coupling capacitor 48, and is power-amplified by the power amplifying transistor 42 and output from the collector thereof. In this case, the amplification gain of the power amplifying transistor 42 becomes a fixed gain, which is dependent on a power voltage applied to the power terminal 57 on the low voltage side. Then, the RF signal which is power-amplified by the power amplifying transistor 42 is supplied to the signal output terminal 55 via the output. filter circuit 50 and the fourth coupling capacitor 51, is output from the signal output terminal 55 through a transmission and receiving select switch (not shown) to an antenna (also not shown) and transmitted.
Meanwhile, the amplification gain of the RF signal in this known gain-control-type transistor power amplifier, namely, the RF signal level output from the signal output terminal 55, depends on the magnitude of the gain control voltage applied to the gain-control-voltage supply terminal 58. In this case, the gain control voltage varies according to the RF signal level (received signal intensity) received by the receiving section of the digital cordless telephone set. When the received signal intensity is large, the gain control voltage decreases. When, on the other hand, the received signal intensity is small, the gain control voltage increases. The varying range is in a range from 0 to 1.5 V.
FIG. 2 is a characteristic view illustrating an example of the relationship between the gain control voltage and the RF signal output level in the gain-control-type transistor power amplifier. FIG. 3 is a characteristic view illustrating an example of the relationship between the gain control voltage and the base voltage of the pre-amplifying transistor in the gain-control-type transistor power amplifier.
In FIG. 2, the horizontal axis indicates the gain control voltage (V) applied to the gain-control-voltage supply terminal, and the vertical axis indicates the RF signal output level (dBm) output from the signal output terminal.
In FIG. 3, the horizontal axis indicates the gain control voltage (V) applied to the gain-control-voltage supply terminal, and the vertical axis indicates the base voltage (V) of the pre-amplifying transistor.
In the above-described known gain-control-type transistor power amplifier, initially, when the gain control voltage applied to the gain-control-voltage supply terminal 58 is from 0 to 0.5 V, as indicated by curve B of FIG. 3, the base voltage of the pre-amplifying transistor 41 does not reach the base-emitter junction voltage (Vbe.apprxeq.0.6 V) of the transistor 41. Therefore, the pre-amplifying transistor 41 is in a cut-off state, and an RF signal is not output, as indicated by curve B of FIG. 2.
Next, when the gain control voltage applied to the gain-control-voltage supply terminal 58 exceeds 0.5 V, as indicated by curve B of FIG. 3, the base voltage of the pre-amplifying transistor 41 increases sequentially with an increase in the gain control voltage, and in response to this, the output level of the RF signal increases sharply, as indicated by curve B of FIG. 2.
At this time, when the gain control voltage applied to the gain-control-voltage supply terminal 58 increases to about 0.8 V, as indicated by curve B of FIG. 3, the base voltage of the pre-amplifying transistor 41 increases sequentially with an increase in the gain control voltage; however, as indicated by curve B of FIG. 2, the output level of the RF signal, which until then was in a sharply increasing state, becomes dull, and in a stage in which the gain control voltage exceeds 0.8 V, the output level of the RF signal saturates with respect to an increase in the gain control voltage thereafter.
Further, when the gain control voltage applied to the gain-control-voltage supply terminal 58 exceeds 1.0 V, although, as indicated by curve B of FIG. 3, the base voltage of the pre-amplifying transistor 41 continues to increase sequentially with an increase in the gain control voltage, as indicated by the curve B of FIG. 2, the output level of the RF signal, which until then was in a saturated state, decreases conversely, and moreover, the rate of decrease is quite sharp.
Thereafter, when the gain control voltage applied to the gain-control-voltage supply terminal 58 increases to about 1.3 V, as indicated by curve B of FIG. 3, the base voltage of the pre-amplifying transistor 41 approaches a saturated state in which the base voltage does not increase greatly with an increase in the gain control voltage, and as indicated by curve B of FIG. 2, the output level of the RF signal becomes almost zero regardless of the gain control voltage.
As described above, the known gain-control-type transistor power amplifier is capable of controlling the output level of the RF signal in a range from -10 to +20 dBm in a range in which the gain control voltage applied to the gain-control-voltage supply terminal 58 is from 0.5 to 0.8 V.
In the above-described known gain-control-type transistor power amplifier, since the output impedance of a control integrated circuit (IC) which supplies a gain control voltage to the gain-control-voltage supply terminal 58 is relatively high, the base bias current based on the gain control voltage supplied to the base of the pre-amplifying transistor 41 cannot be increased. For example, since the base bias current is a maximum of approximately 1 mA, the gain of the pre-amplifying transistor 41 cannot be increased. Therefore, it becomes possible to control the output level of the RF signal only in the range in which the gain control voltage applied to the gain-control-voltage supply terminal 58 is from 0.5 to 0.8 V. There arises the problem that the varying range of the gain control voltage capable of controlling the output level of the RF signal is narrow, and in addition, the linearity of the varying of the output level of the RF signal with respect to the variation in the gain control voltage is not satisfactory.
In addition to this, it is possible that instead of varying the gain of the pre-amplifying transistor 41 by the gain control voltage, the above-described known gain-control-type transistor power amplifier may adopt control means that cause the gain of the power amplifying transistor 42 to be varied or may adopt control means that cause the gain of the pre-amplifying transistor 41 and the gain of the power amplifying transistor 42 in combination to be varied by the gain control voltage. If the former control means is adopted, there arises the problem that the distortion ratio of the RF signal increases during gain control, and spurious components increase. If the latter control means is adopted, there arises the problem that the construction of the circuit which performs gain control becomes complex when there is a difference between the gain of the pre-amplifying transistor 41 and the gain of the power amplifying transistor 42.